One type of solid state image sensor includes a substrate of a semiconductor material, such as single crystalline silicon, having along a surface thereof a plurality of detector areas, such as photodetectors, for detecting radiation and converting the radiation to charge carriers, and transfer means along the detectors for receiving the charge carriers and transferring them to an output terminal. One type of transfer means commonly used is a charge-coupled device (CCD) shift register. Although various types of photodetectors have been used, such as PN diode detectors and Schottky barrier junction detectors, one type which has been found to have certain advantages is a pinned diode. As shown in FIGS. 12, 13 and 14 of U.S. Pat. No. 4,527,182 (Y. Ishihara et al., issued July 2, 1985), entitled "Semiconductor Photoelectric Converter Making Excessive Charge Flow Vertically", a pinned diode comprises a region of N-type conductivity in the surface of the substrate and a region of highly conductive P-type conductivity, generally referred to as P+ type conductivity, in the N-type region and along the surface of the substrate.
One problem which arises when using a pinned diode as the photodetector occurs at the transfer gate between the pinned diode and the CCD shift register. Generally, the transfer gate is a surface channel type gate. A surface channel type gate comprises a region of the substrate, which is generally of P-type conductivity, between an edge of the pinned diode and the buried channel of the CCD shift register forming a transfer channel, and a conductive gate over the transfer channel and insulated there from, generally by a layer of silicon dioxide. In forming a pinned diode, the P+ type region has a tendency to diffuse fully across the N-type region and into the transfer channel of the transfer gate. This results in the formation of a channel potential barrier between the pinned diode and the transfer gate which is higher than that of the balance of the transfer gate channel potential. This type of spurious barrier can result in image lag and increased noise, and thereby reduce the overall image quality. Therefore, it would be desirable to have a transfer gate between a pinned diode photodetector and a CCD shift register of an image sensor which eliminates these problems. Although other types of transfer gates have been used between the photoconductors and a CCD shift register of an image sensor, such as a buried channel transfer gate shown in U.S. Pat. No. 4,774,557 (W. F. Kosonocky, issued Sept. 27, 1988), entitled "Back-Illuminated Semiconductor Imager With Charge Transfer Devices in Front Surface Well Structure", such gates have not been used with pinned diode photodetectors.